How to megger a motor? (Read This First)

How to megger a motor? (Read This First)
megger a motor

What is Motor Megger Test?

Megger test is the process in which we check and calculate internal resistance and the quality of insulation resistance of any electrical device using an instrument called megger.

Insulation resistance test is famous for the term megger related to the well known company MEGGER.

“Megger a Motor” is technically impossible, And you can’t meeger any motors! As megger is a registered trade mark, not an electrical test, But in real life this expression is used when describing “electrical motor insulation resistance test”.

The megger has a direct current generator and an ohmmeter. The usual ohm meter cannot calculate high resistance so, megger is used for this purpose.

To conduct a Megger test we apply a high voltage DC source for a specific time. We can also make sure of electrical safety by using a megger.

What is the purpose of motor megger test?

Insulation tester - megger tester
Megger Ltd., Insulation testers by Megger, CC BY 3.0

Megger testing confines the resistance between the windings and the ground. Megger test check the insulation resistance of the motor winding.

If there is an issue with the insulation, the resistance will be low, the lower the insulation resistance the higher a short circuit chance. The winding insulation protects the motor winding against short circuit between motor winding and motor body.

The electrical system, including the motor’s insulating resistance, decreases over time and with external factors such as humidity, temperature, and dust. It is also negatively influenced by mechanical and electrical stress.

It is critical to test your equipment’s as well as motors Insulation Resistance on a regular basis to guarantee that there is no chance of a deadly electrical shock or a short circuit.

It is commonly assumed that larger resistance is preferable, which is frequently the case. In the best-case scenario, these data are compared over the motor’s service life and utilized to predict approaching failure.

Megger test is important even for new motors. If you have a brand-new induction motor , its insulation should be in very good condition. Whoever, despite motors manufacturers make great improvement for the insulation, but this insulation still susceptible to damage due to corrosion, humidity, moisture, heat, cold and mechanical damage during shipment and transportation.

Even if the motor insulation is in a very good condition in the beginning of its work life, Over time the above mentioned factors effect the insulation resistance making leakage current through the insulation and then may lead to failure.

The good insulation has high resistance while the poor one has low resistance, This resistance decreases gradually so its important to have inspection schedule, and to make megger test and to have good recording the values of the test for each motor.

For more details about motor insulation read my article here.

When Should You Megger a Motor?

  • If you notice that the motor becomes hot and its temperature rises.
  • If you touch the motor and feel electric shock.
  • To determine whether a motor winding insulation is in excellent or bad condition.
  • Before energizing the motor for the first time.
  • Before energizing a motor which was out of service for a long period.
  • Periodically when performing  preventive maintenance program tests.

All the above cases you need to “megger” the motor, testing each of the three phases of the motor to ground and to each other to check if it is shorted to either ground or itself.

It would be best if you determined your Insulation Resistance of the motor through megger tests at regular intervals to avoid any fatalities or electric shocks. A good preventive maintenance program should schedule the interval between tests.

Motor Insulation resistance test (megger test)

The principle of this test is simple, the instrument apply voltage and measure the current then gives you the resistance value.

Never ever perform the megger test over an energized equipment. De energize the motor before the test.

Now, the steps to check different phases and earth resistance, follow the same procedure as described below.

For checking earth resistance, connect the megger probe with the earth terminal and the other probe to the winding, the recorded value is the insulation resistance to earth.

To find the first, second, and third winding resistance, connect the second probe with each winding respectively.

  • A vey high reading, usually in megohms or gigaohms, will indicate a healthy motor insulation.
  • If 0 ohms or overload  is detected your motor may went bad.
  • Use 500 V megger for low voltage motors, up to 600V motors.
  • Use 1000 V megger for medium voltage motors, up to 6600 V motors.

What Voltage Should You Megger a Motor?

Use a Megger with a proper test voltage for the motor in concern, as shown below:

  • A 500 Volt megger is all that is required for low voltage motors, voltage range must be with in the 208 to 600 VAC.
  • A 1000 Volt megger is used with medium voltage motors, voltage range must be with in the 2300-6600 VAC.
  • A 2500 Volt megger is used for high voltage motors, voltage range must be with in the 7kV – 13kV.

What is acceptable megger reading for a motor?

The insulating resistance should be roughly one megohm per 1,000 volts of working voltage.

For example, an electric motor rated at 2,400 volts must have a minimum insulating resistance of 2.4 megohms.

You may understand more about the present status of the insulation if you take readings and take notes on a regular basis. Even if your results are more significant than the specified minimum safe levels, any continuous downward trend is usually an indicator of danger ahead.

Furthermore, if your readings have been consistent over time, you may be safe, even if they are lower than the recommended minimums.

It is worth noting that excellent insulation exhibits a continuous rise in resistance over time (in the order of 5 to 10 minutes).

If the conventional megger test results in a low reading with all three phases connected, separate the three phases and re-check each individual phase to ground.

If the three readings are quite comparable, these low values are most likely not the result of a damaged motor. To drive out leftover moisture, we recommend heating the engine overnight, or for at least 12 hours, with heat lamps, strip heat, or another heat source.

While the windings are still heated, the motor should be re-tested. If the motor passes the tests, the problem was moisture buildup, and remedial measures such as permanent motor space heaters should be recommended.

If there is a significant improvement, but the reading is still not satisfactory, it may be required to heat the motor for a longer period of time.

If the motor never achieves an acceptable value, it should be sent to a repair workshop and baked and re-varnished in a vacuum impregnation tank.

Advantages of Megger Test

  1. Megger test is easy to operate.
  2. The accuracy is up to the mark.
  3. Megger is safe to use, and megger test isn’t destructive.
  4. These are high resistance measuring devices and can detect resistance between zero to an infinite value.
  5. Insulation failure may lead to internal short circuit and burn the equipment.
  6. Periodic megger your equipment and record the insulation resistance value.

Making the megger test ensures the insulation condition of the motor to be suitable for the motor proper operation.

If you perform motor megger test periodically, you could prevent motor insulation failure and a possible short circuit.

One of the greatest advantages of the motor megger test is that it only takes few minutes.

How Long Does a Megger Test Take?

You can run the megger test for only 60 seconds using hand-cranked Megger equipment, getting your first reading at 30 seconds.

If you have a line-powered Megger, run the test for 10 minutes, obtaining data at 1- and 10-minute intervals to calculate the polarization index.

The table below shows the ratios’ values and the relative insulation conditions.

INSULATION CONDITION

60/30 – SEC RATIO

10/1 – MUNITE RATIO

Dangerous

Less than 1

Questionable

1.0 to 1.25

1.0 to 2

Good

1.4 to 1.6

2 to 4

Excellent

Above 1.6

Above 4

These figures should be seen in a restricted and relative context, depending on the outcomes of the time-resistance approach throughout time.

Can a Megger Damage a Motor?

No, a megger test didn’t damage the motor if you perform it correctly, but A high voltage over an extended period of time may further weaken or fail motor windings, and the testing technique may cause winding insulation to be destroyed.

Megohmmeters (meggers) are testing devices that are safe if the user follows the instruction provided by the manufacturer in the instruction manual and electrical safety rules.

The main purpose of a megohmmeter is to identify weak insulation in the motor windings and to determine the accumulation of moisture and acid formations from motor windings before they cause damage to the insulation of the motor windings. If the winding insulation is good, then the megger test will not affect its condition.

What is the difference between hi-pot and megger?

“Megger” tests low and medium voltage, with a range of 600 to 2,000 volts in one minute.

“Hi-pot” tests use a substantially higher voltage, ranging from 11,000 volts to 15,000 volts and up to 300 volts per mil of insulation. “Hi-pot” tests are completed in 15 minutes, with readings values collected every minute.

“Megger” and “hi-pot” (High Voltage Test) tests are often used in the electrical sector to verify the integrity of electrical conductors and other components.

“Megger” is a generic term for a test performed using meg-ohmmeter, and “hi-pot” is an abbreviation for high potential used to identify insulator failure.

While both tests have comparable execution methods, there are notable differences between “megger” and “hi-pot” tests.

Both the “Megger” and “hi-pot” test evaluate insulation resistance, which measures current leakage in conductors.

Can You Megger a Motor With a Multimeter?

No, we cannot megger a motor with a multimeter because, in the megger test, we measure the insulation resistance of the motor winding with respect to the ground, and megger is special testing equipment used to measure the insulation resistance with high voltage generator where High voltage is required to induce a moment of stress in the cable insulation.

On the other hand, multimeters are small, portable testing equipment that measure various electrical quantities such as resistance, voltage, and current with a low-value range compared to megger.

If you think you have a problem with the motor’s windings, If you have a multi-meter, you can check the winding for a short circuit by the buzzer, just put a lead of the multimerter on the winding and the other lead on the motor body, if the motor is good, it shouldn’t buzz.

Keep in mind that this test is an easy method to figure out whether your motor requires megger testing or a complete revoking.

Can You get Shocked by a Megger?

Yes, obviously because the megger supplies over 500 volts, and that is enough to give a fatal shock. It has a Very high internal impedance, so even though the Voltage is high, the current is very low.

Because megger induce high voltage to cable during testing, high voltage is well known as a major workplace danger, exposing personnel to the risk of electrical shock, electrocution, burns, fires, and explosions.

Its important to keep your body away of the tested motor and the megger leads during the test. Also, after the test make sure the megger discharge all the voltages before touching it.

A good megger should include self discharge technique to ensure no voltages remain on the leads after the test end. You can find out if the megger has a self discharge from the user safety guide of the megger.

If the megger has no self discharge, you should contact the manufacturer and ask about the best practice to discharge it manually.

It worth mentioning that you should make a work area around the tested motor and prevent unauthorized persons from getting close.

Why is DC Voltage Applied in an Insulation Resistance Test?

The resistance of the winding, not its impedance, may be used to assess its condition. This is why only DC Voltage is utilized to calculate insulating resistance.

That is, the insulation resistance test examines the resistive component of the insulation.

This capacitive component is not taken into account. The equipment is safe if at least a particular insulating resistance (as determined by an approved Standard) remains. The insulation resistance test is performed to ensure resistance.

When it comes to applying DC voltage, and as we all know, Capacitor blocks the flow of DC, and only leakage current caused by the defect in the insulation will be able to flow, so the resistance that is measured by Megger will be a measure of Insulation Resistance.

This is why the Insulation Tester or Megger applied dc voltage in an insulation resistance test.

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